The invention is directed to a catalytic waste gas converter for internal combustion engines in which there is used as carrier for the catalyst a carrier matrix made of steel screening and which through special flow guides for the engine waste gas to be purified, the steel screen matrix exhausts flow through possibilities inherent in those matrices.
The toxic materials of waste gases especially waste gases from internal combustion motors of the steadily increasing number of motor vehicles represents a considerable danger to the health of men, animals and plants. They are limited in several countries by the legal regulation to maximum concentrations of toxic material. Among the solutions proposed for these air pollution problems and already practiced methods, the catalytic waste gas purification process has acquired the greatest importance. Greatest demands have been placed on the catalysts required therefore in regard to behavior on heating, effectiveness, lasting activity and mechanical stability. For example, when employed in motor vehicles, they must be effective even at the lowest possible temperatures and guarantee a high percentage reaction of the toxic materials to be removed (especially hydrocarbons, carbon monoxide and nitric oxide as well as aldehydes, alcohols and carbon black) to form the non-toxic oxidation and reduction products carbon dioxide, steam and nitrogen for a long time at all temperatures and space velocities which are used. Because of the severe mechanical requirements during the operation, they must possess a sufficient mechanical stability and are permitted not to be lost even with long overheating, as can occur in a given case through being acted on by unburned fuel, for example, in ignition loss in one or more cylinders. Thus, they must satisfy a number of conditions which are difficult to fulfill simultaneously or run contrary to one another.
Previously, besides poured bed catalysts, i.e., or extrudates of catalyst carriers or interspersant or mixed catalysts there were used above all, monolithic catalyst carriers. They consist of an inert, low surface area ceramic skeleton of, e.g. cordierite, mullite or .alpha.-aluminum oxide as structural reinforcer to which there is applied a thin, usually high surface area layer of a heat resistance, usually oxidic carrier material such as aluminum oxide of the so-called gamma series, which latter in turn carries the true catalytically active components. These can consist of noble metals, noble metal compounds or non-noble metal compounds. Of the group of noble metals, there are employed for example platinum, palladium, rhodium, ruthenium, iridium, gold and silver.
As non-noble metal compounds there are employed, e.g. the oxides of copper, chromium, manganese, iron, cobalt, nickel and their combinations as e.g. copper chromite. Further variants are formed by combining noble metals or their compounds with non-noble metals or their compounds or non-noble metals or their compounds with noble metals or their compounds. In many cases there are added to the active components small amounts of other elements, for example, from the group of alkaline earth metals such as magnesium, calcium, strontium or barium, from the group of rare earths, as e.g. samarium, lanthanum, cerium or from the fourth group of the periodic system, as, e.g. titanium, zirconium or tin, as so-called promoters for improving specific properties of the system.
As a considerable disadvantage of the catalyst having ceramic structural reinforcers, especially the monolithic honeycomb catalysts of cordierite, mullite or .alpha.-aluminum oxide, there has proven their poor heat conductivity and their sensitivity to mechanical influences and thermal overheating. Thus the vibrations occurring during traveling through the intermittent impulse of the waste gas columns, the motor vibration and the traveling motions in combination with temperature peaks act to wear down and crumble the ceramic. With thermal overheating in the spatially narrowly limited monoliths there can occur sintering, melting and fusing of the structural reinforcer in the form of monoliths or poured bodies with its coatings from which partial or complete inactivation results.
Furthermore, it has been proven that installing such ceramic honeycombs in metal housings is difficult because of the different thermal expansion of ceramic and metal and requires expensive construction precautions in order to guarantee an elastic and gas tight holding of the honeycomb with the relative motions at the continuously changing operating temperatures in the possible range between -30.degree. and +1000.degree. C.
Therefore, there has been a series of efforts to find better suited replacement materials for the catalyst built on a ceramic basis and to look for a move favorable spatial designing for these.
Thus there has already been described a carrier matrix which is prepared from an alternatively arranged corrugated and smooth high temperature resistant steel sheet which is coated with catalyst. However, in this case, there is the disadvantage that the carrier has a limited geometrical surface which limits to such an extent the supporting capability compared to catalyst carrying, high surface area, heat resistant metal oxides, such as .gamma.-Al.sub.2 O.sub.3, present in immersed dispersion that to produce a sufficiently strong coating on these oxides with the actually catalytically active components there is needed a many times repeated immersion process. Since the known carrier matrices are passed through by flow channels separated from one another, the reacting gas mixture only comes in contact with the catalyst material in the form of individual enclosed, longitudinally flowing gas columns from the walls; through this with predetermined gas flow velocity there is required specific, frequently too large minimum length of the matrix in order to produce a satisfactory exchange of material and connected therewith a sufficient degree of conversion.
Subsequently, there occurs between the individual longitudinally running discrete reaction zones a drop in temperature for example because of local more or less different layer thicknesses and activities of the catalyst material which can only be equalized via the specific heat conductivity of the material of the channel wall.
According to German patent application No. P 2853547.9, and corresponding U.S. Ser. No. 102,581, filed Dec. 11, 1979, now U.S. Pat. No. 4,271,044, the problem of providing carrier matrices for catalysts having flow channels passing therethrough consisting of superimposed layers of high temperature resistant and scale resistant steel, which permits a cross current between the individual flow channels, has an enlarged geometrical surface and shows an improved supporting capability compared to catalyst carrying carrier materials present in immersing dispersions, is solved by constructing the matrix of alternating layers of smooth and corrugated screens (sieve netting) whereby the layers are coiled to a cylinder having a spiral cross section and having numerous flow channels.
In consideration of the fact that in such coils in addition to the normally longitudinally flowing channels of the motor waste gas to be purified, there also is permitted via the openings of the screens a cross current to adjacent channels (so-called cross current effect) there is a need of holding apparatuses for such matrices which permit the utilization of these inherent flow possibilities in a more favorable manner.